Long ago, as a boy, I learned the hard way not to clean coins, when I put an Indian head cent (that was pretty much black) into some copper cleaner. The coin came out bright and shiny, but the surface of the coin was very grainy. But now, as an old man, I've had a little success improving coins that have environmental damage by gently rubbing the affected surfaces with a cotton swab, being careful not to scratch the coin. I've also used a toothpick to remove some "sludge," though it's easy to scratch a coin with a toothpick.
I couldn't tell you how many wheat cents and worn slick Indian head cents I polished with a pencil eraser as a lad..... And I apologize to anyone whose collections those coins ended up in.....
Something else to try that might be better than a toothpick. I've gotten flecks of gunk off with those plastic floss picks. They are sharp but soft, and if the spot is small it works fairly well......sort of like a rose thorn I suppose.
I think one of the problems with cleaning (and advice on cleaning) is that it's easy to think you're doing no damage when you actually are. I kind of cringe when I hear the term "gently rubbing", because yeah, it's possible to "gently rub" a coin without leaving damage, but it's a LOT easier to do it in a way that does leave marks. And even if you're doing it super-gently, there might be some grit on the coin, and if you drag that grit across the surface, you're going to get a scratch. I don't have specific advice myself, because I haven't ruined enough coins yet to know how to avoid doing it...
Aw c'mon man. You know Paddyman has to clean up his buried treasures......it's just a matter of how you go about it.
